1. Field of the Invention
The present invention relates generally to methods for preparing ferrite powders and more particularly to methods for preparing ferrite powders which may be performed at room temperature.
2. Background of the Invention
The prior methods known in the industry for the preparation of ferrite powders are generally based on two different techniques. One technique is the high temperature processing of metal salts or oxides, and the other technique is coprecipitation.
The high-temperature technique for ferrite powder preparation is the traditional ceramic processing method. In this process, metal oxide or metal carbonate powders are mixed together in a ball mill and are then calcined at approximately 1200.degree.-1400.degree. C. to form the crude ferrite. This solid mass is subsequently ground, pressed and sintered at approximately 1200.degree.-1400.degree. C. or higher to form the final, fired parts.
This process suffers from a number of potential drawbacks. The grinding steps may introduce impurities and result in a broad distribution of particle sizes. Further, dry pressing can introduce porosity in the sintered parts. While this method may be acceptable for producing materials for some applications, there are other, more demanding applications that require low impurity content, uniform, small grain size, and low porosity.
For more demanding applications that require low impurity content, uniform, small grain size and low porosity, coprecipitation has been the method of choice. Prior coprecipitation techniques have included the base precipitation of metal hydroxides from sulfate solution and the precipitation of metal oxalates from acetate solutions.
Coprecipitation of ferrites through the base precipitation of metal hydroxides involves dissolution of metal salts (sulfates, halides, nitrates, etc.) in deaerated water. Dissolved oxygen must be removed from the water in order to prevent premature oxidation of Fe(II) to Fe(III). This solution is heated above 60.degree. C. and an aqueous base is added to precipitate the metal hydroxides as a gel. Many bases are effective, including NaOH, and NR.sub.4 OH (R-methyl, ethyl, butyl). Oxygen has been used as the oxidizing agent. Thus, following precipitation, oxygen is bubbled through the reaction mixture, resulting in precipitation of finely dispersed ferrite powder. In general, the ratio of metal ions in the produced powder corresponds to the ratio of metal ions in the initial solution. This process may be summarized in the equation provided in FIG. 1, in which R=CH.sub.3, C.sub.4 H.sub.9 and the O.sub.2 flow rate=4-12 standard liters per hour.
The following is a more rigorous description of the prior art precipitation process using various starting materials and oxygen as the oxidizing agent. Each process occurs in two steps: precipitation of a mixture of hydroxides followed by oxidation to ferrite. In these equations, R is an alkyl, such as methyl, ethyl or butyl, and X is a halide, such as fluoride, chloride, bromide or iodide.
__________________________________________________________________________ sulfates: (1) w MgSO.sub.4 + x MnSO.sub.4 + y ZnSO.sub.4 + z FeSO.sub.4 + 2 NR.sub.4 OH .fwdarw. w Mg(OH.sub.2) (.dwnarw.) + x Mn(OH).sub.2 (.dwnarw.) + y Zn(OH).sub.2 (.dwnarw.) z Fe(OH).sub.2 (.dwnarw.) + (w+x+y+z) (NR.sub.4).sub.2 SO.sub.4 (2) w Mg(OH).sub.2 + x Mn(OH).sub.2 + y Zn(OH).sub.2 + z Fe(OH).sub.2 + z/6 O.sub.2 .fwdarw. Mg.sub.w Mn.sub.x Zn.sub.y Fe.sub.z O.sub.(w+x+y+4z/3) + (w+x+y+z) H.sub.2 O nitrates: (1) w MgNO.sub.3 + x MnNO.sub.3 + y ZnNO.sub.3 + z FeNO.sub.3 + 2 NR.sub.4 OH .fwdarw. w Mg(OH).sub.2 (.dwnarw.) + y Zn(OH).sub.2 (.dwnarw.) + z Fe(OH).sub.2 (.dwnarw.) + (w+x+y+z) (NR.sub.4).sub.2 NO.sub.3 (2) w Mg(OH).sub.2 + x Mn(OH).sub.2 + y Zn(OH).sub.2 + z Fe(OH).sub.2 + z/6 O.sub.2 .fwdarw. Mg.sub.w Mn.sub.x Zn.sub.y Fe.sub.z O.sub.(w+x+y+4z/3) + (w+x+y+z) H.sub.2 O halides: (1) w MgX.sub.2 + x MnX.sub.2 + y ZnX.sub.2 + z FeX.sub.2 + 2 NR.sub.4 OH .fwdarw. w Mg(OH).sub.2 (.dwnarw.) + x Mn(OH).sub.2 (.dwnarw.) + y Zn(OH).sub.2 (.dwnarw.) + z Fe(OH).sub.2 (.dwnarw.) + 2(w+x+y+z) NR.sub.4 X (2) w Mg(OH).sub.2 + x Mn(OH).sub.2 + y Zn(OH).sub.2 + z Fe(OH).sub.2 + z/6 O.sub.2 .fwdarw. Mg.sub.w Mn.sub.x Zn.sub.y Fe.sub.z O.sub.(w+x+y+4z/3) + (w+x+y+z) H.sub.2 O __________________________________________________________________________
This coprecipitation process typically produces a sub-micron powder with narrow particle size distribution and very low impurity content (dependent on the starting materials). The problems with this process pertain to dispersion of the oxygen gas, especially in large volumes of solution, and complete removal of residual base from the precipitate. In particular, dispersion of oxygen in the solution is optimized by using a glass frit or "dispersion stone", which will break the gas stream into many small bubbles. This process creates considerable foaming action in the metal hydroxide gel. When carried out on a large (i.e., greater than 10 liter) scale, the foam can be difficult to contain. Complete removal of base from the product requires extensive washing as the alkyl ammonium hydroxides are not volatile and therefore are not readily removed by evaporation. Since the powders produced by this process are very fine, separation of powder from the liquid phase can be time consuming either by filtration or by centrifugation.
As noted above, another approach to producing ferrites by coprecipitation involves precipitation of a solid solution of metal oxalate salts from acetic acid solution. In this approach, metal acetates are dissolved in refluxing aqueous acetic acid. The addition of oxalic acid results in immediate, quantitative precipitation of the mixed oxalate. This oxalate is converted to the corresponding ferrite by calcining at temperatures greater than 575.degree. C. The calcination step must be carefully controlled to accomplish the conversion to the ferrite without sintering of the particles. This process may be summarized by the equation provided in FIG. 2.
When carefully carried out, this procedure also results in a sub-micron powder with very low impurity content. This process has the drawbacks of requiring high temperatures for calcination and close control of the calcination process. Calcination is not required in the sulfate/hydroxide process.
U.S. Pat. No. 5,078,984 to Iwasaki et al. describes precipitation of doped barium ferrite platelets from a solution of hydroxides. Iwasaki et al. is concerned with making platelets having an average particle size of from 0.03 .mu.m to 0.1 .mu.m, thus, there are several differences between the Iwasaki invention and the present invention. For example, the Iwasaki reference teaches of precipitating the metal hydroxides at pHs of at least 12. Further, Iwasaki et al heat their hydroxide dispersion to temperatures of between 50.degree. C. and boiling and also add a carbonating agent to the reaction mixture. Further, Iwasaki et al. use Fe(III) salts as the iron containing starting material. Significantly, Iwasaki et al. complete the crystallization of their product by calcining at temperatures of between 700.degree. C. and 900.degree. C.
U.S. Pat. No. 4,764,429 to Mair is directed toward preparation of two layer particles, 5-100 nm having a core of Fe.sub.2 O.sub.3 and a shell of a basic metal hydroxide sulfate. While precipitation under the teaching of Mair is also carried out at elevated pH, Mair requires several special steps in order to produce the two layer particles. Thus, the differences between Mair and the present invention include that Mair adds 5-20% by volume of an alcohol to the dispersion of hydroxides prior to isolating the product. The core Fe.sub.2 O.sub.3 particles used in Mair are prepared by the prior art method using NaOH or KOH and an inert gas. Alkali metal hydroxides are undesirable in the device of the present invention because every effort is made to rigorously exclude alkali metals from the product in order to optimize the magnetic properties of the ferrite particles.
Thus, a method for preparing ferrite powders is needed which may be carried out at room temperature and permits the easy removal of the base, preferably by evaporation rather than washing. Such a method would substantially reduce the time and labor involved in the sulfate/hydroxide precipitation method. Such a method must also be able to produce powders with low impurity content, uniform, small grain size and low porosity.